Hypocrisy at its lowest. I dropped this novel around chapter 50, so I will not claim to know what really happened in the novel. But here's my guess anyway. Either you were projecting that mc have romantic feeling toward that girl or he's rational enough to know that she had no obligation to reserve her body only just for him because they were not together. And from the look of it, I think MC was not interested, which render your hypothetical example invalid.

Wait... wait wait! You don't like mc getting ntr but you can tolerate it if the mc ntr someone instead? Furthermore, how could that be considered ntr if they are not even officially together yet? I never understand how people like you(who enjoy harem) think. Thanks god this is not a harem story. I will instantly drop it otherwise.

First of all, you need to stop repeatedly using the same adjectives or adverbs every damn paragraph. Maddening, wildly, immense, just to name some of the most used here. Also, the world building, specifically, the power system DOES NOT make sense at all and very badly thought out. Who in the world thought that just learning skill at that rank make your rank upgrade? Power rank also does not have clear define line like in Wuxia or LitRpg. In LitRpg we got level. In wuxia we got the nice foundation abilities each rank up. The fact that you make rank upgrade by changing mc's skill make him look like a freaking joke.One rank he use these abilities, the next rank, he just dish those and use the new one he got from enemies.